1. Field of the Invention
The present invention relates to a photo-electrical detecting apparatus for forming a dark-field image of an object on a one-dimensional image sensor and photo-electrically reading said image.
2. Description of the Prior Art
A photo-electrical detecting apparatus for photoelectrically reading a dark-field image of an object is already disclosed in the U.S. Pat. Nos. 3,796,497 and 5,062,623. Particularly the latter discloses an apparatus for reading a dark-field image of an object by means of a one-dimensional image sensor such as a one-dimensional image pick-up tube, a chargecoupled device or a photo-detector array.
The present invention relates to an improvement on the apparatus disclosed in said U.S. Pat. No. 4,062,623, said improvement being featured in that the object is illuminated with a line- or band-shaped light beam. In case of reading an object by means of a one-dimensional image sensor, an illumination of the entire surface of the object is quite useless, and it is desirable to illuminate only a line- or band-shaped area which is to be read by said image sensor. In such case, however, the illuminating system is required to satisfy the condition of forming a dark-field image on a one-dimensional image sensor, i.e. conditions of employing a telecentric arrangement for the objective lens and forming an image of illuminating light source on the plane of aperture of said objective lens. A telecentric objective lens is a lens the front focal point of which coincides with the position of the entrance aperture thereof. However, in case of the apparatus disclosed in the aforementioned U.S. Pat. No. 4,062,623 wherein the image of illuminating light source formed on the plane of aperture is in focus in two perpendicular directions, the illuminated area on the object becomes two-dimensionally spread.